The present invention relates to the production of composites and is particularly concerned with an epoxy resin system applicable for use in the pultrusion process for fabricating composite structures especially useful in the aircraft and aerospace industries.
Structural components particularly advantageous for aircraft, as well as other vehicles, e.g., in the form of beams, ribs and other structural components, including "J" stiffeners, "C" channels, and "I" beams, are often made of lightweight material relative to their strength and stiffness. Such materials are composite materials which, as is well known in the art, are comprised of fibers of various types, such as graphite fibers in cloth and/or tape orientations, impregnated with a binder material, usually a plastic, such as an epoxy resin.
Presently, the manufacture of composite structures, such as leading and trailing edges, stiffeners and wing and tail structures, in the aircraft industry involves expensive labor intensive processes. While it is desirable to reduce such costs, the problem is that most aerospace applications involve low volume and "one of a kind" design criteria, which are not readily amenable to automation. Thus, although automation technologies have been used in other industries, they have not been applied to any substantial extent in the aerospace industry.
Various technologies have been examined for their applicability to aerospace composite structure manufacturing. These include injection molding, thermoplastic forming processes, transfer molding and pultrusion. These technologies have been used extensively in the automotive, computer/electronic and construction industries. Thermoplastic forming techniques using polycarbonate and similar non-structural thermoplastics have been widely used in commercial aircraft interiors.
High temperature thermoplastic forming is now being employed fairly extensively in the aerospace industry. The leading thermoplastics, polyether ether ketone (PEEK) and polyphenylene sulfide (PPS) in such process have high forming temperatures, 750.degree. F. and 650.degree. F., respectively, which require expensive high temperature tools and equipment. In addition, their high melt viscosities make processing more difficult. These materials can be formed more readily using injection molding. However, this process is only feasible for small, high volume parts since the cost of injection molding equipment is quite expensive.
Resin transfer molding is a technology which has not been developed as yet to any substantial degree in the aerospace industry. This process does not require expensive initial costs for equipment, as with injection molding, and can be an inexpensive method for producing one-of-a-kind composite structures.
However, pultrusion is an ideal process for producing constant cross-section members, such as C-channels, I-beams, hat sections, J-sections and the like, in an automatic manner. The pultrusion process involves the steps of creeling long runs of fabric or individual fiber tows of glass, graphite or Kevlar fiber, such as graphite roving, to order the various layers of fabric, collating or assembling the various layers together, wetting out the collated fabric with a suitable impregnating resin in wet-out tanks, forming the assembly of resin impregnated tows or fabrics by passing through shaping forms and then passing the resulting formed resin impregnated fabric assembly through a pultrusion die. The die is heated to a predetermined temperature along the length of the die to effect curing of the resin during passage through the die. In the pultrusion die, a pulling together of the fibers or layers of the fabric occurs, and the curing of the resin in the die forms a pultruded cross-section.
Although the pultrusion process is an attractive economic automatic procedure which permits the production of structural parts and complex shapes, particularly in the aerospace industry, at a lower cost, and affords less fiber distortion, thus increasing structural strength, the selection of matrix resins suitable for pultrusion has presented a significant problem. Polyester resins, vinyl esters and certain epoxy resins have been tried in the pultrusion process. However, all of these resins have not exhibited the mechanical properties applicable to aerospace applications.
Epoxies as a group have the disadvantage of requiring long cure cycles to achieve acceptable engineering properties, and epoxies tend to expand through gellation. Successful pultrusion as a process requires very fast kinetic rates of resin cure (less than three minutes), long pot life at elevated temperature and viscosity profile windows low enough to wet out fibers but not high enough to flow away from fibers in the pultrusion die.
It is accordingly an object of the present invention to provide a viable epoxy resin system for use in a pultrusion process for production of composites.
Another object is the provision of an improved epoxy resin system for use in a pultrusion process, having fast cure rate, long pot life at elevated temperature and suitable viscosity profile, and providing a highly reactive predictable reaction rate, resulting in low cost production of composites having good mechanical properties.
A still further object is the provision of an improved pultrusion process for producing epoxy composite structures, particularly in the aircraft industry, rapidly and at low cost.